The stability of sodalite in the system NaAlSiO4-NaCl

The reaction sodalite = β-nepheline + NaCl ( s ) was reversed in solid-medium apparatus (923 K, 7.6–8.0 kbar; 973 K, 7.4–7.6 kbar; 1073 K, 7.8–8.0 kbar; 1173 K, 8.4–8.6 kbar) and the reaction sodalite = carnegieite + NaCl ( l ) was reversed at 1 bar (1649–1652 K). The calculated slope for the first reaction is −11 bar/K compared to the experimentally constrained slope above 973 K of +5 bar/K. This major discrepancy can be eliminated if the entropy of sodalite (Na 8 Al 6 Si 6 O 24 Cl 2 ) is 61.7 J/mol · K greater than the measured value of 848.1 ± 4.0 J/mol · K at 298.15 K. This entropy contribution is equal to that expected from 58% disorder of Al and Si on the tetrahedral sites, but could also be caused by a reversible phase change. The experimental reversals between 923 K and 973 K can be fit with a dP dT of −11 bar/K, suggesting that the excess entropy for sodalite is present only above 923 K. A phase diagram for the NaAlSiO 4 -NaCl system that is consistent with the measured thermochemical data and the experiments between 973 and 1650 K can be generated if the 61.7 J/mol · K entropy contribution is included in the S 298 0 of sodalite. This entropy contribution must be removed below 973 K for the experiments to fit with calculations. Previously unreported thermodynamic data estimated in this study are ΔG 298 0 for sodalite (−12697 kJ/mol) and carnegieite (NaAlSiO 4 ) (−1958 kJ/mol), S 298 0 of carnegieite (129.6 J/mol · K) and compressibility of NaCl liquid ( V 298 P (cm 3 ) = 31.6 · [1 − 24.7 · 10 −3 · P + 800 · 10 −6 · P 2 ]) ( T in K; P in kbar). Sodalite is a high-temperature, low-pressure phase, stable well above the solidus in sodic silica-undersaturated magmas enriched in NaCl, and its presence constrains NaCl activities in magmas. Estimates of minimum NaCl ( l ) activities in the Mont St-Hilaire sodalite syenites are 0.05 at 1073 K and 0.13 at 1273 K. Density calculations are consistent with the field observations that sodalite phenocrysts will float in a nepheline syenite liquid. This explains the enrichment of sodalite in the upper levels of the sodalite syenites at Mont St-Hilaire and elsewhere.